Method and apparatus for extracting optical clock signal

ABSTRACT

An apparatus and method for extracting an optical clock signal are provided. The apparatus includes a first reflection filter selecting and reflecting only a first frequency component in an input optical signal; a first Fabry-Perot laser diode matching the first frequency component reflected by the first reflection filter with a predetermined output mode and outputting the first frequency component in the predetermined output mode; a second Fabry-Perot laser diode selecting a second frequency component in an input optical signal that has not been reflected but has been transmitted by the first reflection filter, matching the second frequency component with a predetermined output mode, and outputting the second frequency component in the predetermined output mode; and a photodetector receiving the first frequency component from the first Fabry-Perot laser diode and the second frequency component from the second Fabry-Perot laser diode and beating them to extract a clock signal. Accordingly, the optical clock signal can be extracted with low influence of the pattern of the input optical signal and an improved signal-to-noise ratio (SNR).

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This applicationThe present patent application is a Reissue of U.S. Pat.No. 7,577,370, issued on Aug. 18, 2009, which claims the benefit ofKorean Patent Application No. 10-2005-0096947, filed on Oct. 14, 2005,in the Korean Intellectual Property Office, the disclosuredisclosures ofeach of which isare incorporated herein in itstheir entirety byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and apparatus for extractingan optical clock signal, and more particularly, to a method andapparatus for extracting an optical clock signal with reduced influenceof the pattern of an input optical signal by using characteristics of aFabry-Perot laser diode.

2. Description of the Related Art

With the increase of transmission speed in optical communication and thedevelopment of technology of a transmitter converting data into anoptical signal, the increase of a signal processing rate of a receiver,which receives the optical signal and recovers it to the original data,has been required. To satisfy the request, a method and apparatus forextracting an optical clock signal have been studied.

To extract an optical clock signal, a method using a self-pulsatinglaser diode, a method using an optical loop mirror, a method using anoptical tank circuit, etc. have been studied. However, it is stilldifficult to manufacture an optical element for extracting a desiredclock signal and an optical system is still unstable.

To overcome these problems, a method of recovering a clock signal usinga frequency component existing in an optical spectrum has beensuggested. In other words, adjacent two frequency componentscorresponding to the data transmission rate of a received optical signalare extracted and beating is performed thereon to generate a frequencycomponent corresponding to a difference between two spectral lines, sothat a clock signal is recovered.

In the above-described conventional method, two frequency components areselected in an optical spectrum and made to have the same intensity.Thereafter, beating is performed on the two frequency components,thereby obtaining a clock signal for an optical signal. To select twofrequency components and make them have the same intensity, aconventional method illustrated in FIG. 1 is used.

FIG. 1 illustrates a conventional system for extracting an optical clocksignal using a tunable band-pass filter 120. Referring to FIG. 1, inorder to make first and second frequency components or second and thirdfrequency components have the same intensity in an input frequencyspectrum 110, the intensity of the second frequency component should bedecreased.

For an nonreturn-to-zero signal, the input optical signal with the inputfrequency spectrum 110 is passed through the tunable band-pass filter120. The tunable band-pass filter 120 performs appropriate attenuationon frequency components of the input optical signal, thereby making thefirst and second frequency components or the second and third frequencycomponents have the same intensity. In detail, the tunable band-passfilter 120 puts the first or third frequency component at a point P1giving the least attenuation and puts the second frequency component ata point P2 giving the most attenuation to make the first and secondfrequency components or the second and third frequency components havethe same intensity. Reference numeral 130 denotes the characteristic ofthe tunable band-pass filter 120.

Here, a difference between the intensity of the first frequencycomponent and the intensity of the second frequency component or betweenthe intensity of the second frequency component and the intensity of thethird frequency component must be similar to a difference betweenattenuation at the point P1 and attenuation at the point P2 in thetunable band-pass filter 120 to make the first and second frequencycomponents or the second and third frequency components have the sameintensity within an error range. When a difference between the intensitydifference and the attenuation difference is great, the methodillustrated in FIG. 1 is not efficient. In other words, the tunableband-pass filter 120 suitable to the characteristics of an opticalspectrum of an input optical signal needs to be used or the tunableband-pass filter 120 needs to be specially manufactured to be suitableto the characteristics of the optical spectrum of the input opticalsignal. Reference numeral 140 denotes an optical spectrum of the opticalsignal that has passed through the tunable band-pass filter 120.

Moreover, in the method illustrated in FIG. 1, an extracted clock signalis greatly influenced by the pattern of an input optical signal. Inother words, when data of the input optical signal is continuously “0”or “1”, a clock signal component may disappear.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for extracting anoptical clock signal, in which the intensity of a side-peak frequencycomponent and the intensity of a central frequency component are madethe same by controlling current or temperature of a Fabry-Perot laserdiode, thereby greatly decreasing the influence of the pattern of aninput optical signal and increasing a signal-to-noise ratio (SNR).

According to an aspect of the present invention, there is provided anapparatus for extracting an optical clock signal. The apparatus includesa first reflection filter selecting and reflecting only a firstfrequency component in an input optical signal; a first Fabry-Perotlaser diode matching the first frequency component reflected by thefirst reflection filter with a predetermined output mode and outputtingthe first frequency component in the predetermined output mode; a secondFabry-Perot laser diode selecting a second frequency component in aninput optical signal that has not been reflected but has beentransmitted by the first reflection filter, matching the secondfrequency component with a predetermined output mode, and outputting thesecond frequency component in the predetermined output mode; and aphotodetector receiving the first frequency component from the firstFabry-Perot laser diode and the second frequency component from thesecond Fabry-Perot laser diode and beating them to extract a clocksignal.

According to another aspect of the present invention, there is provideda method extracting an optical clock signal. The method includes theoperations of (a) selecting and reflecting only a first frequencycomponent in an input optical signal; (b) matching the first frequencycomponent reflected in operation (a) with a predetermined output mode ofa first Fabry-Perot laser diode and outputting the first frequencycomponent in the predetermined output mode; (c) selecting a secondfrequency component in the input optical signal that has not beenreflected in operation (a) but has been transmitted, matching the secondfrequency component with a predetermined output mode of a secondFabry-Perot laser diode, and outputting the second frequency componentin the predetermined output mode; and (d) beating the first frequencycomponent obtained in operation (b) and the second frequency componentobtained in operation (c) to extract a clock signal.

According to another aspect of the present invention, there is provideda computer readable recording medium for recording a program forexecuting the method in a computer.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill become more apparent by describing in detail exemplary embodimentsthereof with reference to the attached drawings in which:

FIG. 1 illustrates a conventional circuit for extracting an opticalclock signal using a tunable band-pass filter;

FIG. 2 is a block diagram of an apparatus for extracting an opticalclock signal according to an embodiment of the present invention;

FIG. 3 is a block diagram of an apparatus for extracting an opticalclock signal according to another embodiment of the present invention;

FIG. 4 is a block diagram of an apparatus for extracting an opticalclock signal according to still another embodiment of the presentinvention; and

FIG. 5 is a flowchart of a method of extracting an optical clock signalaccording to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to the attached drawings.

FIG. 2 is a block diagram of an apparatus for extracting an opticalclock signal according to an embodiment of the present invention.Referring to FIG. 2, the apparatus includes a first circulator 200, afirst reflection filter 210, a first Fabry-Perot laser diode 220, asecond circulator 230, a second Fabry-Perot laser diode 240, a coupler250, a photodetector 260, and a controller 270.

The first reflection filter 210 selects and reflects only a firstfrequency component in an input optical signal and transmits otherfrequency components.

The first circulator 200 receives an input optical signal from an inputterminal of the circulator and circulates the input optical signal sothat the input optical signal is input to the first reflection filter210. In addition, the first circulator 200 circulates the firstfrequency component reflected from the first reflection filter 210 sothat the first frequency component is input to the first Fabry-Perotlaser diode 220.

The first Fabry-Perot laser diode 220 receives the first frequencycomponent reflected by the first reflection filter 210, controls currentor temperature applied thereto to match the first frequency componentwith its predetermined output mode, and then outputs the first frequencycomponent to the first circulator 200.

The first circulator 200 receives the first frequency component matchedwith the predetermined output mode from the first Fabry-Perot laserdiode 220 and circulates the first frequency component to output it tothe coupler 250.

The frequency components transmitted by the first reflection filter 210are input to the second circulator 230. The second circulator 230circulates the input optical signal from which the first frequencycomponent has been removed so that the input optical signal without thefirst frequency component is input to the second Fabry-Perot laser diode240.

The second Fabry-Perot laser diode 240 receives the input optical signalwithout the first frequency component from the second circulator 230,controls current or temperature applied thereto to match the inputoptical signal without the first frequency component with itspredetermined output mode, and then selects and outputs a secondfrequency component to the second circulator 230.

The second circulator 230 receives the second frequency componentmatched with the predetermined output mode from the second Fabry-Perotlaser diode 240 and circulates the second frequency component to outputit to the coupler 250.

The coupler 250 receives the first frequency component and the secondfrequency component from the first circulator 200 and the secondcirculator 230, respectively, couples them, and outputs the coupledfirst and second frequency components to the photodetector 260.

The photodetector 260 performs beating of the coupled first and secondfrequency components, thereby extracting a clock signal.

The controller 270 individually controls the first Fabry-Perot laserdiode 220 and the second Fabry-Perot laser diode 240 to make the firstfrequency component and the second frequency component have the sameintensity. Here, the controller 270 controls current or temperatureapplied to each of the first and second Fabry-Perot laser diodes 220 and240 to make the intensity of the first frequency component the same asthe intensity of the second frequency component.

In addition, the controller 270 enables only a particular frequencycomponent to be selected in an input optical signal by controllingcurrent or temperature applied to each of the first and secondFabry-Perot laser diodes 220 and 240.

In an optical spectrum, the intensity of a side-peak frequency componentis less than that of a central frequency component. In this situation,to maximize the intensity of a clock signal obtained by beating the twofrequency components against the intensity of an ambient noisecomponents, it is necessary to make the two frequency components havethe same intensity. Referring to FIG. 2, two frequency components areselected and made to have the same intensity by the operation of thecontroller 270 controlling current or temperature applied to the firstFabry-Perot laser diode 220 and the second Fabry-Perot laser diode 240.

Here, the first frequency component may be one among side-peak frequencycomponents of the input optical signal and the second frequencycomponent may be the central frequency component of the input opticalsignal. Since the side-peak frequency component has less intensity thanthe central frequency component, the side-peak frequency component maybe input to the first Fabry-Perot laser diode 220 having a shorteroptical path than the second Fabry-Perot laser diode 240 to minimize thedecrease of the intensity over the optical path.

In addition, a non-return-to-zero (NRZ) optical modulation signal may beused as the input optical signal. Since the first circulator 200, thefirst reflection filter 210, the first Fabry-Perot laser diode 220, thesecond circulator 230, and the second Fabry-Perot laser diode 240 can beused only for an optical signal, an NRZ electrical signal cannot be usedas it is. In an embodiment of the present invention, the fact that anoptically modulated NRZ signal has a side-peak frequency component isused. In other words, a side-peak frequency component is not present inan NRZ electrical signal but is present in an NRZ, optical modulationsignal, and therefore, the NRZ optical modulation signal is used as theinput optical signal.

FIG. 3 is a block diagram of an apparatus for extracting an opticalclock signal according to another embodiment of the present invention.Referring to FIG. 3, the apparatus includes a first circulator 200, afirst reflection filter 210, a second reflection filter 215, a firstFabry-Perot laser diode 220, a second circulator 230, a secondFabry-Perot laser diode 240, a coupler 250, a photodetector 260, acontroller 270, a first band-pass filter 280, and a second band-passfilter 285.

With respect to members denoted by the same reference numerals in FIGS.2 and 3, the above description may be referred to. Hereinbelow, newmembers, i.e., the second reflection filter 215, the first band-passfilter 280, and the second band-pass filter 285 will be described.

Frequency components of an input optical signal transmitted by the firstreflection filter 210 are input to the second circulator 230.

The second reflection filter 215 selects and reflects a second frequencycomponent in the input optical signal from which a first frequencycomponent has been removed.

The second circulator 230 circulates the input optical signal from whichthe first frequency component has been removed to output it to thesecond reflection filter 215. In addition, the second circulator 230circulates the second frequency component reflected from the secondreflection filter 215 to output it to the second Fabry-Perot laser diode240.

The second Fabry-Perot laser diode 240 receives the second frequencycomponent from the second circulator 230, matches the second frequencycomponent with its predetermined output mode according to current ortemperature applied thereto by the controller 270, and then outputs thesecond frequency component to the second circulator 230.

The second circulator 230 receives the second frequency componentmatched with the predetermined output mode from the second Fabry-Perotlaser diode 240 and circulates the second frequency component to outputit to the coupler 250.

The first band-pass filter 280 is disposed between the first circulator200 and the coupler 250 and removes noise components from the firstcirculator 200 together with the first frequency component. After thenoise components are removed, the first band-pass filter 280 alsooutputs the first frequency component to the coupler 250.

The second band-pass filter 285 is disposed between the secondcirculator 230 and the coupler 250 and removes noise components from thesecond circulator 230 together with the second frequency component.After the noise components are removed, the second band-pass filter 285outputs the second frequency component to the coupler 250.

The reason why the noise components are removed using the firstband-pass filter 280 and the second band-pass filter 285 will bedescribed below. Each of the first Fabry-Perot laser diode 220 and thesecond Fabry-Perot laser diode 240 outputs a frequency component in oneoutput mode among a plurality of output modes according to current ortemperature applied thereto by the controller 270. However, frequencycomponents are also output in other output modes as well as a desiredoutput mode in the first Fabry-Perot laser diode 220 and the secondFabry-Perot laser diode 240. To remove output components in output modesother than the desired output mode, the first band-pass filter 280 andthe second band-pass filter 285 are used. When the first band-passfilter 280 and the second band-pass filter 285 are used, asignal-to-noise ratio (SNR) is increased.

FIG. 4 is a block diagram of an apparatus for extracting an opticalclock signal according to still another embodiment of the presentinvention. Referring to FIG. 4, the apparatus includes a firstcirculator 200, a first reflection filter 210, a second reflectionfilter 215, a first Fabry-Perot laser diode 220, a second circulator230, a second Fabry-Perot laser diode 240, a coupler 250, aphotodetector 260, a controller 270, a first band-pass filter 280, asecond band-pass filter 285, a first polarization controller 290, and asecond polarization controller 295.

With respect to members denoted by the same reference numerals in FIGS.2 through 4, the above description may be referred to. Hereinbelow, newmembers, i.e., the first polarization controller 290 and the secondpolarization controller 295 will be described.

The first polarization controller 290 is disposed between an inputterminal of the apparatus and the first circulator 200 and controls onlya particular polarization component in an input optical signal to beinput to the first reflection filter 210.

The second polarization controller 295 is disposed between the firstreflection filter 210 and the second circulator 230 and controls only aparticular polarization component in the input optical signal to beinput to the second circulator 230.

When the first polarization controller 290 and the second polarizationcontroller 295 are used, only particular polarization component is inputto each of the first Fabry-Perot laser diode 220 and the secondFabry-Perot laser diode 240, so that the SNR can be increased.

FIG. 5 is a flowchart of a method of extracting an optical clock signalaccording to an embodiment of the present invention. Referring to FIG.5, an input optical signal is input to a reflection filter, whichselects and reflects only a first frequency component, in operationS510.

In operation S520, it is determined that a corresponding frequencycomponent has been reflected. The determination is made such that thefirst frequency component desired to be reflected is reflected and theinput optical signal from which the first frequency component has beenremoved is transmitted.

When it is determined that the corresponding frequency component hasbeen reflected in operation S520, that is, when the first frequencycomponent is input, the intensity of the first frequency component isadjusted using a first Fabry-Perot laser diode in operation S530.Thereafter, in operation S540, noise components existing together withthe intensity adjusted first frequency component are removed using afirst band-pass filter.

When it is determined that the corresponding frequency component has notbeen reflected in operation S520, that is, when the first frequencycomponent is not input, a second frequency component is extracted fromthe input optical signal in operation S525.

In operation S535, the intensity of the second frequency component isadjusted using a second Fabry-Perot laser diode. In operation S545,noise components existing together with the intensity adjusted secondfrequency component are removed using a second band-pass filter.

After operation S540 or S545, the first frequency component is coupledwith the second frequency component in operation S550. Next, inoperation S560, a clock signal is extracted by beating the coupled firstand second frequency components.

Here, the first frequency component may be one among side-peak frequencycomponents of the input optical signal and the second frequencycomponent may be the central frequency component of the input opticalsignal. Since the side-peak frequency component has less intensity thanthe central frequency component, the side-peak frequency component maybe input to the first Fabry-Perot laser diode having a shorter opticalpath than the second Fabry-Perot laser diode to minimize the decrease ofthe intensity over the optical path.

In addition, an NRZ optical modulation signal may be used as the inputoptical signal. Since the reflection filter and the first and secondFabry-Perot laser diodes can be used only for an optical signal, an NRZelectrical signal cannot be used as it is. In an embodiment of thepresent invention, the fact that an optically modulated NRZ signal has aside-peak frequency component is used. In other words, a side-peakfrequency component is not present in an NRZ electrical signal but ispresent in an NRZ optical modulation signal, and therefore, the NRZoptical modulation signal is used as the input optical signal.

The invention can also be embodied as computer readable codes on acomputer readable recording medium. The computer readable recordingmedium is any data storage device that can store data which can bethereafter read by a computer system. Examples of the computer readablerecording medium include read-only memory (ROM), random-access memory(RAM), CD-ROMs, magnetic tapes, floppy disks, optical data storagedevices, and carrier waves (such as data transmission through theInternet). The computer readable recording medium can also bedistributed over network coupled computer systems so that the computerreadable code is stored and executed in a distributed fashion.

While the influence of the pattern of an input optical signal is greatin conventional method and apparatus for extracting an optical clocksignal, the influence can be greatly reduced due to the characteristicsof a Fabry-Perot laser diode in the present invention. In addition,according to the present invention, current or temperature applied tothe Fabry-Perot laser diode is controlled to maintain the amplitude of aclock signal constant, so that the SNR of the clock signal can beincreased.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the present invention as defined by the following claims.

What is claimed is:
 1. An apparatus for extracting an optical clocksignal, the apparatus comprising: a first reflection filter selectingand reflecting only a first frequency component in an input opticalsignal; a first Fabry-Perot laser diode matching the first frequencycomponent reflected by the first reflection filter with a predeterminedoutput mode and outputting the first frequency component in thepredetermined output mode; a second Fabry-Perot laser diode selecting asecond frequency component in an input optical signal that has not beenreflected but has been transmitted by the first reflection filter,matching the second frequency component with a predetermined outputmode, and outputting the second frequency component in the predeterminedoutput mode; and a photodetector receiving the first frequency componentfrom the first Fabry-Perot laser diode and the second frequencycomponent from the second Fabry-Perot laser diode and beating them toextract a clock signal.
 2. The apparatus of claim 1, further comprisinga controller individually controlling the first Fabry-Perot laser diodeand the second Fabry-Perot laser diode to make the first frequencycomponent and the second frequency component have the same intensity. 3.The apparatus of claim 2, wherein the controller controls current ortemperature applied to each of the first Fabry-Perot laser diode and thesecond Fabry-Perot laser diode.
 4. The apparatus of claim 1, furthercomprising: a first circulator receiving and transmitting the inputoptical signal to the first reflection filter, transmitting the firstfrequency component reflected by the first reflection filter to thefirst Fabry-Perot laser diode, and transmitting the first frequencycomponent output from the first Fabry-Perot laser diode to thephotodetector; and a second circulator transmitting the input opticalsignal that has not been reflected but has been transmitted by the firstreflection filter to the second Fabry-Perot laser diode and transmittingthe second frequency component output from the second Fabry-Perot laserdiode to the photodetector.
 5. The apparatus of claim 4, furthercomprising a second reflection filter disposed between the secondcirculator and the second Fabry-Perot laser diode, the second reflectionfilter receiving the input optical signal from the second circulator,selecting the second frequency component in the input optical signal,and reflecting the second frequency component to the second Fabry-Perotlaser diode.
 6. The apparatus of claim 4, further comprising: a firstband-pass filter disposed between the first circulator and thephotodetector to remove noise components from the first circulatortogether with the first frequency component; and a second band-passfilter disposed between the second circulator and the photodetector toremove noise components from the second circulator together with thesecond frequency component.
 7. The apparatus of claim 1, furthercomprising: a first polarization controller controlling only aparticular polarization component in the input optical signal to beinput to the first reflection filter; and a second polarizationcontroller disposed between the first reflection filter and the secondFabry-Perot laser diode to control only a particular polarizationcomponent in the input optical signal to be input to the secondFabry-Perot laser diode.
 8. The apparatus of claim 1, further comprisinga coupler receiving the first frequency component from the firstFabry-Perot laser diode and the second frequency component from thesecond Fabry-Perot laser diode, coupling them, and outputting thecoupled first and second frequency components to the photodetector. 9.The apparatus of claim 1, wherein the first frequency component is onecomponent among side-peak frequency components of the input opticalsignal.
 10. The apparatus of claim 1, wherein the second frequencycomponent is a central frequency component of the input optical signal.11. A method extracting an optical clock signal, the method comprisingthe operations of: (a) selecting and reflecting only a first frequencycomponent in an input optical signal; (b) matching the first frequencycomponent reflected in operation (a) with a predetermined output mode ofa first Fabry-Perot laser diode and outputting the first frequencycomponent in the predetermined output mode; (c) selecting a secondfrequency component in the input optical signal that has not beenreflected in operation (a) but has been transmitted, matching the secondfrequency component with a predetermined output mode of a secondFabry-Perot laser diode, and outputting the second frequency componentin the predetermined output mode; and (d) beating the first frequencycomponent obtained in operation (b) and the second frequency componentobtained in operation (c) to extract a clock signal.
 12. The method ofclaim 11, wherein the first frequency component obtained in operation(b) and the second frequency component obtained in operation (c) have asame intensity.
 13. The method of claim 12, wherein the intensity of thefirst frequency component obtained in operation (b) and the intensity ofthe second frequency component obtained in operation (c) are adjusted bycontrolling current or temperature applied to the first Fabry-Perotlaser diode and the second Fabry-Perot laser diode, respectively. 14.The method of claim 11, further comprising: removing noise componentstogether with the first frequency component in operation (b) and goingto operation (d); and removing noise components together with the secondfrequency component in operation (c) and going to operation (d).
 15. Themethod of claim 11, wherein operation (d) comprises: coupling the firstfrequency component obtained in operation (b) with the second frequencycomponent obtained in operation (c); and beating the coupled first andsecond frequency components to extract the clock signal.
 16. The methodof claim 11, wherein the first frequency component is one componentamong side-peak frequency components of the input optical signal. 17.The method of claim 11, wherein the second frequency component is acentral frequency component of the input optical signal.
 18. The methodof claim 11, wherein the input optical signal is a no-return-to-zero(NRZ) optical modulation signal.
 19. An apparatus comprising: a firstreflection filter configured to reflect a first frequency component inan input optical signal; a first Fabry-Perot laser diode configured tomatch the reflected first frequency component with a first predeterminedoutput mode and output the matched first frequency component; a secondFabry-Perot laser diode configured to match a second frequency componentin the input optical signal with a second predetermined output mode andoutput the matched second frequency component; and a photodetectorconfigured to receive the matched first frequency component from thefirst Fabry-Perot laser diode and the matched second frequency componentfrom the second Fabry-Perot laser diode, and beat the received frequencycomponents to extract a clock signal.
 20. A method comprising:reflecting a first frequency component in an input optical signal;matching the reflected first frequency component with a firstpredetermined output mode; matching a second frequency component in theinput optical signal with a second predetermined output mode; andbeating the matched first frequency component and the matched secondfrequency component to extract a clock signal.
 21. The apparatus ofclaim 19, further comprising a controller configured to individuallycontrol the first Fabry-Perot laser diode and the second Fabry-Perotlaser diode to make the first frequency component and the secondfrequency component have the same intensity.
 22. The apparatus of claim21, wherein the controller is configured to control at least one ofcurrent and temperature applied to each of the first Fabry-Perot laserdiode and the second Fabry-Perot laser diode.
 23. The method of claim20, wherein the first frequency component is matched in a firstFabry-Perot laser diode and the second frequency component is selectedin a second Fabry-Perot laser diode.
 24. The method of claim 23, furthercomprising individually controlling the first Fabry-Perot laser diodeand the second Fabry-Perot laser diode and making the first frequencycomponent and the second frequency component have the same intensity.25. The method of claim 24, wherein the intensity of the first and thesecond frequency component is adjusted by controlling at least one ofcurrent and temperature applied to the first Fabry-Perot laser diode andthe second Fabry-Perot laser diode, respectively.